Service providers are committed to providing the type of connectivity their customers require. As a result, the presence of Frame Relay (FR), Asynchronous Transfer Mode (ATM), or Point-to-Point (PPP) technologies on the customer side of the network is not uncommon. These feeds usually connect to a multi-service switch/router.
Ethernet is increasingly being used to interconnect customer equipment through provider networks. The high-speed uplink, however, is often ATM, Packet over Synchronous Optical Network (POS), or Gigabit Ethernet. This demand for various types of connectivity creates many challenges. A primary challenge is mapping the data correctly from one type of technology to another without traffic loss or data-integrity problems. Another challenge involves meeting service guarantees to the customer to meet the applications' requirements.
The legacy Ethernet Standard and devices support only a single QoS per interface. Similarly FR Standards do not support multiple QoS levels per connection.
The Institute of Electrical and Electronics Engineers (IEEE) Standard 802.1Q Ethernet Specification, however defines a tag, inserted into Ethernet frames, that defines virtual-LAN (VLAN) membership. Three bits in this tag identify user priority as defined by IEEE 802.1Q to provide for up to eight priority levels. Switches and routers can, therefore, use the tag to give traffic precedence by queuing outgoing frames in multiple buffers.
Similarly, Diff-Serv is an Internet Engineering Task Force (IETF) specification that works at the network layer by altering the Internet protocol (IP) type-of-service field to identify particular classes of service, The Internet Engineering Task Force (IETF) is a large open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet. Diff-Serv could be used for signaling the class of service per Ethernet frame when the Upper Layer Protocol (ULP) is IP. Diff-Serv, however, is simply a class-of-service management scheme rather than a complete QoS mechanism.
Other internetworking protocols available for supporting QoS include: Resource Reservation Protocol Traffic Engineering (RSVP-TE), used to reserve end-to-end network resources for a particular network flow (in one direction); Real-Time Transport Protocol, which is optimized to deliver real-time data such as audio and video streams through multiplexed User Datagram Protocol (UDP) links; IP Multicast; and Multi-protocol Label Switching (MPLS).
The Metro Ethernet Forum (MEF) stipulates the use of the IEEE 802.1Q tag and/or the layer 3 (L3), and higher layer, fields in the packet header to support multiple QoS on an Ethernet interface. The most common application among networking providers is when L3 traffic is IP with Diff-Serv.
Frame relay networks are connection-oriented and can support QoS using two mechanisms. The first relates to bandwidth guarantee through a preset Committed Information Rate (CIR) and Excess Information Rate (EIR) that allow packets to be designated as either “committed” or “excess”. Unlike CIR packets that are guaranteed low loss rate, EIR packets receive low or no bandwidth guarantee. The other mechanism assigns a QoS class or transfer/emission priority to the FR connection. Note that unlike an Ethernet VLAN that can support multiple QoS using the p-bits, a Standard FR connection can only support a single QoS.
Frame Relay (FR) traffic management involves a discard eligibility (DE) bit that is available in the Q922 header to indicate the discard priority of a frame. A frame with DE=1 will be discarded first upon congestion. FR standards allow for the specification of 16 transfer priorities (TP) and 8 discard priorities (DP). Transfer and discard priorities are applicable to all frames that belong to a particular FR Data Link Connection (DLC). The DLC transfer and discard priorities are defined by ITU X.36 where 16 TP levels are specified. TP is defined per DLC and for each direction. During data transfer a DLC with a high TP will have its frames serviced before frames of a DLC with a lower TP. Eight DP levels are specified. DP is defined per DLC and for each direction. Upon network congestion, frames with a lower DP level will be discarded in preference to frames with a higher DP level. Frame TP and frame DP are set at subscription time or by signaling or configuration. FR standards also specify an alternate method for QoS signaling/configuration per DLC and support of four services (ITU-T X.146) differentiated by frame loss and frame delay expectations.
When choosing a frame relay service, customers typically assign voice/video traffic to a low delay connection, and allocate sufficient CIR bandwidth to minimize packet discards. Data connections are typically assigned to lower QoS classes/priorities, depending on the applications' needs. The range of QoS parameters are specified in the Frame Relay Forum's FRF.13 Service Level Definition. This specifies the service level agreements and details how delay, frame delivery, data delivery, and service availability are calculated.